A semiconductor element mounted on a semiconductor substrate includes electrodes electrically connected to a semiconductor substrate having external terminal electrodes, and a bonding material used for bonding is generally a solder material. In die bonding of the semiconductor element, attention should be given to the elemental components of a die bonding material. When die-bonding the semiconductor element, it is necessary to select a die bonding material which is electrically connected to the semiconductor substrate without inclining the semiconductor element or causing voids.
The semiconductor device is mounted on a motherboard with another bonding material that is generally a solder material having a melting point of 200° C. to 230° C.
When the semiconductor device is mounted on the motherboard, the semiconductor device is prepared to be fixed on a predetermined position on the motherboard with the solder material and the semiconductor device is heated with the motherboard mainly by reflow equipment of a hot-air system to melt the solder material having a melting point of 200° C. to 230° C. At this point, the temperature of the semiconductor device reaches 230° C. to 260° C. When the solder material for bonding the semiconductor element and semiconductor substrate electrodes is melted in the semiconductor device, a movement of the solder material causes an unexpected electrical connection which may result in a failure. Thus the bonding material used in the semiconductor device is required to have a higher melting temperature than the maximum temperature reached by the semiconductor device in the reflow equipment. Further, it is necessary to prevent the generation of an intermetallic compound which reduces the melting point of the die bonding material, and thus it is important to select a material according to a temperature and the number of processes. Moreover, attention should be given to exfoliation caused by mechanical strength degradation on a joint.
A solder material containing tin as a composition forms an intermetallic compound layer with a metal of a bonded material, and the intermetallic compound layer changes with time and causes mechanical strength degradation on a joint.
Thus in a semiconductor device of the prior art, a solder material for bonding a semiconductor element and semiconductor substrate electrodes is, for example, a Pb—Sn alloy which contains lead as a main component, contains about 3 wt % of Sn, and has a melting temperature of 319° C.
However, of die bonding materials for a semiconductor element of the prior art, lead-tin (Pb—Sn) solder and tin-antimony (Sn—Sb) solder contain lead (Pb) and antimony (Sb) as metals and are not expected to be used as die bonding materials in the future. Thus lead-tin solder and tin-antimony solder are not proper as die bonding materials. Particularly, when lead is used, the lead may be released to soil from a solder material in waste. In recent years, there has been a growing interest in global environmental protection and solder not containing lead (lead-free solder) has been developed. For example, solder materials composed of a Pb—Sn alloy having a melting temperature of 200° C. to 250° C. have been replaced with solder materials composed of one of a Sn—Ag alloy and a Sn—Cu alloy. As a solder material having a melting temperature of at least 260° C., a solder material containing Bi as a main component and a small amount of Ag has been proposed.
However, solder using silver (Ag) and gold (Au), which are expensive metals, is an expensive die bonding material and thus increases the production cost. Lead-indium (Pb—In) solder contains lead (Pb) and has a low melting point of 173° C., so that Pb—In solder is not proper as a die bonding material of a semiconductor element for power in which heat is generated during an operation.
Thus a solder material for electronic components has been proposed in which a bonding material containing an alloy predominantly composed of Bi is used. The alloy contains 0.2 wt % to 0.8 wt % of Cu and one of 0.02 wt % to 0.2 wt % of Ge and 0.02 wt % to 0.08 wt % of Ni.